Usually, directional sensitivity in, for example, hearing aids is achieved by using (i) matched pairs of two omnidirectional microphones or (ii) analogue directional microphones.
Using omnidirectional microphones, directional hearing in hearing aids is normally achieved by the use of a matched pair of two omnidirectional microphones. Two operational modes exist: directional and omnidirectional mode. In the directional mode, the signals of both microphones are subtracted. An electrical time delay is applied to one of the signals. In the omnidirectional mode, either only one of the microphones is used or the signals of both microphones are added, which leads to a 3 dB better SNR.
Instead of using omnidirectional microphones, directional hearing in a hearing aid can also be achieved by the use of an analogue directional microphone. An analogue directional microphone is a microphone with a second sound inlet in the rear volume, wherein one of the sound inlets has an acoustical filter to achieve a time delay. The membrane only detects pressure differences between the front and the rear sound inlet. Therefore, the analogue directional microphone only works in directional mode. The advantage of an analogue directional microphone is that directionality cannot be degraded by drift over time.
These types of systems have advantages and disadvantages. For example, matched pairs of two omnidirectional microphones typically have the following characteristics:                Double space and energy consumption of an omnidirectional microphone.        If the sensitivity and/or phase of the two microphones of a matched pair drift away from each other over time by aging effects or on shorter time scales due to environmental influences directional performance in the low frequencies degrades quickly.        Low signal-to-noise-ratio in directional mode in the low frequencies which makes it necessary to switch to omnidirectional mode in quiet situations.And, analogue directional microphones typically have the following characteristics:        The delay has to be made with acoustic filters such as external tubing or grids and cannot be changed. Therefore, directionality can only be static (no dynamic beam forming).        Low signal-to-noise-ratio in the low frequencies. Switching to omnidirectional mode not possible; thus requiring an additional omnidirectional microphone.        
Examples of systems of the above types may be seen in U.S. Pat. No. 7,245,734 and U.S. Pat. No. 6,788,796.